The present invention relates generally to communication systems and more particularly to a method and apparatus for determining which of a plurality of redundant buses, which concurrently carry the same data between multiple communication devices, is to be utilized by a communications device for the reception of data.
In the discipline of data communications, it is common to connect a plurality of various communication devices to a common bus which provide data links between those devices. It is also known to use plural buses (normally two, but sometimes more) in a redundant manner for reliability purposes. In these redundant systems, the same data is applied to each of the buses and a selection is made between the buses for the receipt of data. When a device transmits data, the device simultaneously puts the data on all of the buses for receipt by the other device(s).
In such a redundant system, a decision must be made as to which bus to use for the receipt of data since the data propagation time along the individual buses is not synchronized and hence an attempt to receive data for more than one bus could have disastrous results. It is a common practice in these systems of the prior art to have some higher authority, such as an overall system supervisor, to inspect the buses and to direct the various communications devices which are to receive the data as to which bus to employ. Several problems exist with respect to this type of system. A first of these problems is that the higher authority making this decision may already be heavily loaded in the number of functions it must perform and this bus selection requirement adds another function to this potentially overloaded authority. In addition, selection of the bus for use by a higher authority may result in the initial loss of fairly large amounts of data. This is because the authority may be involved in other functions at the time a bus develops a fault and thus the need for change is not immediately recognized and due to the fact that once a problem is recognized, the need for a change must be communicated to the other devices on the bus.
A third problem which can exist in the higher authority bus selection scheme being discussed is that if the problem or fault develops not on the bus per se, but on the leads from the bus to the communications device, the higher authority may not necessarily recognize that a problem exists. Failing to recognize that a problem exists, the higher authority will not, of course, direct a change to a different bus.